This invention relates to a sputtering target used in sputtering to deposit a high resistivity thin film for forming resistor elements of a circuit, in the manufacture of a high-density wiring board suited to information processing systems such as electronic computers, electronic switching systems, etc.
High resistivity thin films consisting of chromium (Cr), silicon (Si) and oxygen (O) have been known to have a low temperature coefficient of resistance, be stable both thermally and chemically, and be of great use for forming thin film resistors. As a means for producing such a thin film, there is known a method of depositing the desired thin film by heating a mixture of a Cr powder and a silicon monoxide (SiO) powder with electron beams, as disclosed in J. Vac. Sci. Technol., 4 (1967), pp. 163-170.
Also, it is described, for example in Japanese Patent Application Laid-Open Nos. 54-1898 (1979) and 58-82770 (1983), that the aforementioned thin film can be obtained by reactive sputtering in an oxygen-containing atmosphere using a Cr-Si composite-type target or alloy target.
Furthermore, there is known an example of depositing the aforementioned thin film by using a mixture of a Cr powder and an SiO.sub.2 powder directly as a sputtering target, as described in Thin Solid Films, 57 (1979), pp. 363-366.
Although different from the above exemplary methods in regard to the material system for forming the thin film, there is also known an example of using a sputtering target formed by pressing metallic tantalum (Ta) and SiO.sub.2 powders and carrying out sputtering to obtain a thin film consisting of tantalum (Ta), silicon (Si) and oxygen (O), which is a known example of a metal-silicon-oxygen thin film, as is disclosed in Japanese Patent Application Laid-Open No. 58-119605 (1983).
Of the prior art examples mentioned above, those methods for depositing a thin film consisting of Cr, Si and O are, in the short run, satisfactory for obtaining the desired thin film. The methods, however, are not satisfactory with respect to reappearance characteristic of film characteristics in the long run, and lack considerations as to stability with time of obtaining the thin film; therefore, these methods tend to cause deviations of concentrations of film constituents, and are unsatisfactory for industrial use. On the other hand, in the example of a sputtering target formed from metallic Ta and SiO.sub.2, the forming must be carried out at a temperature as low as possible, because high temperatures would cause reactions between tantalum and silicon and/or between tantalum and oxygen, resulting in a very brittle target. The requirement for forming at a low temperature leads to a low relative density of particles for forming thin film particles. Even with various countermeasures against this problem, the density of the thin film forming particles has been about 90% at best. Consequently, the presence of minute voids in the sputtering target obtained will cause variations in the distribution of voids on the surface and in the thickness direction of the target. Thus, the sputtering targets according to the prior art have problems yet to be solved as to reappearance characteristic (stability with time) on a long-term basis.
In addition, the presence of such greater voids as to be visible to the naked eye would cause localization of glow discharge during sputtering, thereby rendering the discharge unstable.
In order to obviate this problem, the relative density of the thin film forming particles should be raised to 90% or above. Use of a high pressure in pressing for forming the sputtering target, in order to obtain the high relative density, produces another problem that heat generation due for example to friction between particles during the pressing makes it impossible to hinder the progress of the aforementioned reactions. Therefore, the target material obtained in this manner is extremely brittle, and highly susceptible to cracking under the heat arising from electric discharge in deposition of a thin film by sputtering.